1. Field of the Invention
The present invention relates to a fuel cell system.
2. Description of Related Art
In recent years, a development of a fuel cell such as a polymer electrolyte fuel cell (hereinafter, referred to as PEFC), which generates electricity by being supplied hydrogen to the anode and air (oxidant gas) containing oxygen to the cathode, is more popular than before. The fuel cell described above includes an anode flow path (fuel gas flow path) through which the hydrogen is supplied to and discharged from the anode flows, and a cathode flow path (oxidant gas flow path) through which the air is supplied to and discharged from the cathode flows.
However, if a shutdown of the fuel cell continues, impurities (for example, nitrogen crossed over electrolyte membrane) may be increased in the anode flow path. Therefore, when the fuel cell is started after the shutdown, the anode may become short of hydrogen, thereby resulting in insufficient performance of electric power generation of the fuel cell.
Therefore, to avoid the above issue, a technology for starting electric power generation of a fuel cell has been proposed (see Japanese Patent No. 2735396, [0011]), in which impurities in the anode flow path is discharged when the fuel cell is started (before starting electric power generation of fuel cell), and the impurities are exchanged for hydrogen by newly supplying hydrogen to the anode flow path from, for example, a hydrogen tank. After that, when OCV (open circuit voltage) of the fuel cell becomes not less than a predetermined OCV by the exchange of the impurities, the electric power generation of the fuel cell is started.
On the other hand, if the fuel cell is exposed in a low temperature environment (for example, below freezing point) during the shutdown, a remaining moisture on a surface or inside MEA (membrane electrode assembly), which composes the fuel cell, may freeze, and thereby an effective reaction cross section (anode and cathode areas contributable to electrode reaction) of the MEA may be narrowed. If the fuel cell is permitted to start generation of electricity under the conditions that the effective reaction cross section of the MEA is narrowed as described above based on the OCV not less than a predetermined OCV, the MEA may become short of the gas, and thereby a performance of the electric power generation and stability of the fuel cell may be lowered.
Especially, the shortage of the gas tends to take place when a current output (output current) from the fuel cell is increased for increasing self-heating to promote warming up of the fuel cell since the fuel cell might freeze. It is noted that a start-up with rapid increase in temperature to a preferable temperature for the electric power generation (for example, 80 to 90° C. for PEFC) by promoting warming up of the fuel cell is called a cold start.
It is, therefore, an object of the present invention to provide a fuel cell system which hardly becomes short of a fuel gas in the MEA after staring generation of electricity when the fuel cell system is started with the cold start even if the effective reaction cross section of the MEA is narrowed.